A true triaxial condition rock fracture toughness testing device and testing method

Abstract

The application discloses a true triaxial condition rock fracture toughness testing device and testing method, and relates to the technical field of rock mechanics and engineering; the testing device comprises a lateral confining pressure loading system located in a loading chamber; the loading chamber is located between a pressurizing base and a cover plate, and the three form a sealed space; hydraulic oil is continuously filled into the sealed space to apply confining pressure on a rock test piece in the vertical direction; an upper loading column is installed at the bottom of the cover plate, and the upper loading column is in contact with the rock test piece as a fracture fulcrum; a vertical load loading plate is arranged on the pressurizing base, and a lower loading column is fixed on the vertical load loading plate; the lower loading column is lifted under the drive of a hydraulic drive to apply fracture load on the rock test piece; the application can restore the three-direction stress state of the rock under the ground, test the rock fracture toughness under different three-direction confining pressure conditions, and provide important parameter support for the compressibility evaluation of unconventional oil and gas resources and the fracturing engineering design.

Description

Technical Field

[0001] This invention relates to the field of rock mechanics and engineering technology, specifically to a testing device and method for testing rock fracture toughness under true triaxial conditions. Background Technology

[0002] Shale oil, tight oil, and other unconventional oil and gas reservoirs are characterized by low porosity and low permeability. Before development, these resources require large-scale hydraulic fracturing to create a network of underground fractures, ensuring sufficient oil and gas production for commercial development. Horizontal well combined with volumetric fracturing is expensive. To avoid indiscriminate fracturing, it is usually necessary to evaluate the reservoir's fracturing potential before development—a process known as fracturing capability assessment. As an inherent property of rock, fracture toughness characterizes its ability to prevent fracture propagation. The magnitude of fracture toughness relates to the ease with which fractures extend; a lower value indicates easier fracture propagation, which is more conducive to hydraulic fracturing. Therefore, accurately obtaining fracture toughness values ​​can provide precise parameters for unconventional reservoir fracturing capability assessment models.

[0003] Fracture toughness originates from materials mechanics. To more accurately measure the fracture toughness of materials, scholars both domestically and internationally have proposed many methods for testing fracture toughness, such as the circular beam method (CB), the short bar method (SR), and the Brazilian disk method (CCNBD). With the increasing demand for unconventional oil and gas resource development, fracture toughness has been introduced into the evaluation of reservoir fracturing capability. Due to the high success rate of specimen preparation and the simplicity of experimental operation, the three-point bending test of two semi-discs, the straight groove specimen (NSCB) and the herringbone groove specimen (CCNSCB), has become a commonly used method for testing the fracture toughness of unconventional reservoir rocks. However, current fracture toughness tests are all conducted under normal pressure conditions, which cannot reflect the influence of underground confining pressure on fracture toughness. The fracture toughness of rocks measured in the laboratory is 1 to 2 orders of magnitude smaller than that obtained from in-situ micro-fracturing. In recent years, some scholars have modified the pseudo-triaxial compressive strength testing instrument for rocks to test the fracture toughness of rocks under different confining pressure conditions. The results show that there is a linear relationship between rock fracture toughness and confining pressure, and the fracture toughness of rocks under different confining pressure conditions may differ by an order of magnitude. However, unconventional reservoirs exhibit typical anisotropic characteristics, experiencing triaxial stress conditions with significant differences in fracture toughness across these directions. The aforementioned pseudo-triaxial fracture toughness testing method applies uniform confining pressure, thus failing to reflect the variations in rock fracture toughness across different stress directions. Since the fracture toughness specimens fail in an opening-and-expansion manner, with varying deformation in different directions, this cannot be achieved using current rigid true triaxial stress methods. Therefore, testing rock fracture toughness under different triaxial stress conditions has become a significant challenge in the exploration and development of unconventional oil and gas resources.

[0004] In summary, there is an urgent need for a testing device and method to obtain the fracture toughness of rocks under true triaxial conditions, so as to narrow the gap between indoor tests and field measurements and provide reliable experimental basis for fracturing design. Summary of the Invention

[0005] The purpose of this invention is to provide a true triaxial rock fracture toughness testing device and method, which recreates the triaxial stress state of rocks underground and tests the fracture toughness of rocks under different triaxial confining pressures. This provides important parameter support for the evaluation of the compressibility of unconventional oil and gas resources and the design of fracturing engineering. Furthermore, by using a pressurization method for opening and expanding fracture specimens, the testing accuracy is improved and the testing error is reduced.

[0006] To achieve the above objectives, the technical solution of this application is as follows: a rock fracture toughness testing device under true triaxial conditions, comprising a pressure base, a loading chamber, a cover plate, and a transverse confining pressure loading system. The transverse confining pressure loading system is located within the loading chamber, which is located between the pressure base and the cover plate, forming a sealed space. Hydraulic oil is continuously injected into the sealed space to apply longitudinal confining pressure to the rock specimen. An upper loading column is installed at the bottom of the cover plate, which serves as a fracture fulcrum and contacts the rock specimen. A longitudinal load loading plate is provided on the pressure base, and a lower loading column is fixed on the longitudinal load loading plate. The lower loading column is raised under hydraulic drive to apply a fracture load to the rock specimen.

[0007] Furthermore, the lateral confining pressure loading system is used to apply lateral loads to the rock specimen, and includes a minimum stress loader and a maximum principal stress loader. The minimum stress loader is symmetrically arranged on the left and right sides of the rock specimen to apply the minimum stress of the underground reservoir to the rock specimen; the maximum principal stress loader is symmetrically arranged on the front and rear sides of the rock specimen to apply the maximum stress of the underground reservoir to the rock specimen.

[0008] Furthermore, the minimum stress loader includes a fixed pad and a flexible ultra-thin jack. The fixed pad is fixedly connected to the inner wall of the loading chamber, and the flexible ultra-thin jack is installed between the rock specimen and the fixed pad.

[0009] Furthermore, the maximum principal stress loader includes a fixed pad, a flexible ultra-thin jack, and a deformation sliding controller. The fixed pad is fixedly connected to the inner wall of the loading chamber. After the flexible ultra-thin jack expands under hydraulic drive, the stress is applied to the deformation sliding controller, thereby transmitting it to the rock specimen.

[0010] Furthermore, the deformation sliding controller has a double-layer foldable structure, with the two layers in contact via ball bearings. The inner layer is connected to the rock specimen and deforms as the rock specimen opens and breaks, while the outer layer is connected to the flexible ultra-thin jack and does not deform.

[0011] Furthermore, the outer layer structure is connected to the top of the inner layer structure via a rotating shaft, and the inner layer structure is fastened to the rock specimen via a fixing strip.

[0012] Furthermore, a longitudinal displacement sensor is installed at the bottom of the cover plate, and a strain gauge is installed on the rock specimen. The longitudinal displacement sensor and strain gauge are connected to an external data acquisition system via a data transmission line to measure the longitudinal and transverse deformation of the rock specimen when it fractures in real time.

[0013] Furthermore, the loading chamber is equipped with a pressure valve and a pressure relief valve, both of which are connected to an external hydraulic system.

[0014] This application also provides a true triaxial rock fracture toughness testing method, the steps of which are as follows:

[0015] Step 1. Attach strain gauges to the top of the pre-cut groove on the rock specimen, wrap the outside with heat shrink tubing, and use a hot air blower to shrink and tightly wrap the rock specimen;

[0016] Step 2. Connect the fixing pad to the inner wall of the loading chamber, which is fixed to the pressure base;

[0017] Step 3. Install the rock specimen into the deformation sliding controller and place it on the two lower loading columns. Insert the flexible ultra-thin jack between the rock specimen and the corresponding fixing pad.

[0018] Step 4. Connect the hydraulic lines of the flexible ultra-thin jack and the data transmission lines of the strain gauges to the external data acquisition system, and then install the cover plate onto the loading chamber.

[0019] Step 5. Start the hydraulic station of the pressurization base to make the upper surface of the rock specimen contact the upper loading column, and then adjust the strain gauge and longitudinal displacement sensor to 0.

[0020] Step 6. First, start the hydraulic station connected to the flexible ultra-thin jack in the direction of maximum stress to apply a load in the direction of maximum stress to the rock specimen. Then, start the hydraulic station connected to the flexible ultra-thin jack in the direction of minimum stress to apply a load in the direction of minimum stress.

[0021] Step 7. Start the hydraulic station connected to the loading chamber. After the hydraulic oil is filled into the loading chamber, apply longitudinal confining pressure to the rock specimen. After the longitudinal confining pressure stabilizes, start the test, monitor the stress-strain curve, and obtain the rock fracture toughness based on the test data after the specimen fractures.

[0022] By adopting the above technical solution, the present invention can achieve the following technical effects:

[0023] 1. In fracture toughness tests, since the rock undergoes an open-extension fracture, the deformation in the three directions is not uniform. Existing testing equipment cannot apply true triaxial stress to it. This invention applies pressure to the triaxial stress and fracture load of the rock specimen using four different loading methods. It can obtain the fracture toughness of the rock under different triaxial confining pressure conditions. The accurate measurement of true triaxial fracture toughness can provide important parameter support for drilling engineering, mining engineering and reservoir stimulation engineering, and has high application value.

[0024] 2. This invention uses a rigid three-point loading column pressurization and a hydraulic oil pressurization in the loading chamber to apply longitudinal fracture load and longitudinal confining pressure to the rock specimen, respectively. Although the two pressurization methods are in the same direction, they do not affect each other, realizing the process of simultaneous pressurization and failure in fracture toughness testing.

[0025] 3. During the three-point fracture process of rock, due to the opening tear failure of the rock specimen, rigid loading will cause significant friction between the rock surface and the loading plate surface, thereby reducing the success rate and accuracy of the test. In this invention, a double-layer foldable deformation sliding controller is installed in the direction of maximum stress of the rock specimen. Its inner layer structure can deform with the rock, while the outer layer structure is fixed to the flexible ultra-thin jack and does not deform. There are ball bearings between the inner and outer layers. While applying load, the friction between the inner and outer layers is greatly reduced, and the measurement accuracy of true triaxial rock fracture toughness is improved. Attached Figure Description

[0026] Figure 1 A three-dimensional schematic diagram of a rock fracture toughness testing device under true triaxial conditions;

[0027] Figure 2 Side view of a rock fracture toughness testing device under true triaxial conditions;

[0028] Figure 3 Top view of a rock fracture toughness testing device under true triaxial conditions;

[0029] Figure 4 This is a schematic diagram of a deformation sliding controller and rock fracture.

[0030] The numbers in the diagram are explained as follows: 1. Pressure base; 2. Loading chamber; 3. Cover plate; 4. Lateral confining pressure loading system; 5. Longitudinal load loading plate; 6. Pressure pipeline; 7. Decompression pipeline; 8. Lower loading column; 9. Rock specimen; 10. Long bolt; 11. Pressure valve; 12. Decompression valve; 13. Upper loading column; 14. Longitudinal displacement sensor; 15. Minimum stress loader; 16. Maximum principal stress loader; 17. Fixing pad; 18. Flexible ultra-thin jack; 19. Deformation sliding controller; 20. Inner structure; 21. Outer structure; 22. Rotating shaft; 23. Fixing strip; 24. Ball bearing. Detailed Implementation

[0031] The technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. It should be understood that the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the protection scope of this invention.

[0032] Example 1

[0033] like Figure 1-4 As shown, this embodiment provides a rock fracture toughness testing device under true triaxial conditions, including a pressure base 1, a loading chamber 2, a cover plate 3, and a transverse confining pressure loading system 4. A longitudinal load loading plate 5 is installed on the pressure base 1. The longitudinal load loading plate 5 is connected to an external hydraulic station by a pressure pipeline 6 and a pressure relief pipeline 7. Two lower loading columns 8 are fixed on the longitudinal load loading plate 5. Under hydraulic drive, the longitudinal load loading plate 5 rises, causing the lower loading columns 8 to apply a splitting load to the rock specimen 9. The loading chamber 2 is a combined structure with the pressure base 1 and the cover plate 3, and is fixed by long bolts 10. A pressure valve 1 is installed on the loading chamber 2. 1 and 12 are connected to an external hydraulic system. During the test, the pressure valve 11 is opened and the pressure relief valve 12 is closed, so that the loading chamber 2 is filled with hydraulic oil to apply longitudinal confining pressure to the rock specimen 9. The bottom of the cover plate 3 is provided with an upper loading column 13 and two longitudinal displacement sensors 14. The upper loading column 13 presses against the middle of the upper surface of the rock specimen 9 to apply a fracture fulcrum. The longitudinal displacement sensors 14 are used to measure the longitudinal deformation of the rock specimen 9 when it fractures. The transverse confining pressure loading system 4 is used to apply transverse loads to the rock specimen 9. It includes two minimum stress loaders 15 and two maximum principal stress loaders 16. The minimum stress loader 15 is symmetrically arranged on both sides of the rock specimen 9 to apply the minimum stress of the underground reservoir to the rock specimen 9. It includes a fixed pad 17 and a flexible ultra-thin jack 18. The fixed pad 17 is fixedly connected to the inner wall of the loading chamber 2. The flexible ultra-thin jack 18 is installed between the rock specimen 9 and the fixed pad 17 and is connected to an external hydraulic station by a pipeline. During the test, the flexible ultra-thin jack 18 expands under hydraulic drive, applying the minimum stress to the rock specimen 9. The maximum principal stress loader 16 is used to apply the maximum stress of the underground reservoir to the rock specimen 9. It includes a fixed pad 17, a flexible ultra-thin jack 18 and a deformation sliding controller 19. After the flexible ultra-thin jack 18 expands under hydraulic drive, the stress is applied to the deformation sliding controller 19, thereby transmitting it to the rock specimen 9.

[0034] The aforementioned deformation sliding controller 19 is a double-layer foldable structure, including an inner layer structure 20 and an outer layer structure 21, which are connected by a rotating shaft 22. The inner surface of the inner layer structure 20 is in contact with the rock specimen 9 and is fixed to the rock specimen 9 by four fixing strips 23 on the top and bottom. Therefore, when the rock specimen 9 expands under a three-point load, the inner layer structure 20 will undergo the same deformation around the rotating shaft 22. The outer layer structure 21 is connected to a flexible ultra-thin jack 18 on the outside and has a seven-layer ball bearing 24 structure on the inside to reduce the friction between the two layers and ensure the continued application of stress load when the rock specimen 9 expands.

[0035] The aforementioned true triaxial rock fracture toughness testing device employs three different load application methods to provide triaxially unequal stress loads to the rock specimen. In the longitudinal direction, hydraulic oil is used to directly pressurize the specimen; this fluid pressurization method can be coordinated with three-point bending loads, allowing for simultaneous fracture and pressurization along the longitudinal direction of the rock. Minimum stress is applied using a flexible ultra-thin jack, which reduces the impact of uneven deformation during rock fracture on stress loading. Maximum stress is applied using a flexible ultra-thin jack in conjunction with a deformation sliding controller, avoiding the influence of rock opening and misalignment on maximum stress pressure. This device achieves accurate measurement of rock fracture toughness under true triaxial stress conditions. The specific testing steps are as follows:

[0036] Step 1. Attach strain gauges to the top of the pre-cut groove on the rock specimen, wrap the outside with heat shrink tubing, and use a hot air blower to shrink and tightly wrap the rock specimen;

[0037] Specifically, irregular underground rocks were cut into 30mm*30mm*60mm cuboids using CNC wire cutting. A 15mm long pre-cut groove was cut in the middle of the rock using a 1mm thick diamond cutting saw blade. Three strain gauges were attached in parallel to the top of the pre-cut groove to test the lateral deformation of the rock specimen when it fractured. Then, heat shrink tubing was wrapped around the outside of the cuboid specimen with the groove, and a hot air blower was used to shrink and tighten it around the rock specimen to complete the preparation of rock specimen 9.

[0038] Step 2. Connect the fixing pad to the inner wall of the loading chamber, which is fixed to the pressure base;

[0039] Specifically, four fixing blocks 17 are fixedly connected to the inner wall of the loading chamber 2, and the insertion holes of the loading chamber 2 are inserted into the long bolts 10 on the base, so that the pressure base 1 is fixed and connected to the loading chamber 2;

[0040] Step 3. Install the rock specimen into the deformation sliding controller and place it on the lower loading column. Insert the flexible ultra-thin jack between the rock specimen and the corresponding fixing pad.

[0041] Specifically, after installing the rock specimen 9 onto the deformation sliding controller 19, it is fixed by the eight fixing strips 23 of the inner structure 20. Then, the ball bearings 24 of the outer structure 21 are adjusted to ensure free sliding. According to the test requirements, the distance between the two lower loading columns 8 on the longitudinal load loading plate 5 is adjusted, the valve of the pressurization line 6 on the pressurization base 1 is opened, and the valve of the depressurization line 7 is closed. The rock specimen 9 with the deformation sliding controller 19 installed is placed on the two lower loading columns 8, and two flexible ultra-thin jacks 18 in the minimum stress direction are inserted between the rock specimen 9 and the fixing pad 17. Then, the position of the specimen is finely adjusted. Then, the flexible ultra-thin jacks 18 in the maximum stress direction are inserted between the deformation sliding controller 19 and the fixing pad 17.

[0042] Step 4. Connect the hydraulic lines of the flexible ultra-thin jack and the data transmission lines of the strain gauges to the external data acquisition system, and then install the cover plate onto the loading chamber.

[0043] Specifically, the hydraulic lines of the flexible ultra-thin jack 18 and the data transmission lines of the strain gauges are connected to the interface of the cover plate 3. The data transmission of the longitudinal displacement sensor 14 and the strain gauges is debugged to ensure their normal operation. Then, the cover plate 3 is installed on the loading chamber 2 and the nuts of each long bolt 10 are tightened. This completes the installation of the rock fracture toughness testing device under true triaxial conditions.

[0044] Step 5. Start the hydraulic station of the pressurization base to make the upper surface of the rock specimen contact the upper loading column, and then adjust the strain gauge and longitudinal displacement sensor to 0.

[0045] Specifically, under hydraulic drive, the longitudinal load loading plate 5 is gradually raised, and the pressure data of the computer is observed. When the longitudinal stress exceeds 0, it indicates that the upper surface of the rock specimen 9 is in contact with the upper loading column 13. At this time, the hydraulic station connected to the pressure base 3 is stopped, and the longitudinal stress and sensor are adjusted to 0.

[0046] Step 6. First, start the hydraulic station connected to the flexible ultra-thin jack in the direction of maximum stress to apply a load in the direction of maximum stress to the rock specimen. After the stress state stabilizes, start the hydraulic station connected to the flexible ultra-thin jack in the direction of minimum stress to apply a load in the direction of minimum stress to the rock specimen 9.

[0047] Step 7. Start the hydraulic station connected to the loading chamber. After the hydraulic oil is filled into the loading chamber, apply longitudinal confining pressure to the rock specimen. After the longitudinal confining pressure stabilizes, start the test, monitor the stress-strain curve, and obtain the rock fracture toughness based on the test data after the specimen fractures.

[0048] Specifically, after the loads in the four horizontal directions have stabilized, the pressure relief valve 12 of the loading chamber 2 is closed, and the hydraulic station connected to the pressure valve 11 is started to gradually fill the loading chamber 2 with hydraulic oil, applying a longitudinal load to the rock specimen 9. After the longitudinal confining pressure has stabilized, the hydraulic station connected to the pressure base 1 is started, and the external data acquisition system is turned on to record the longitudinal stress, longitudinal deformation, and transverse deformation data at each time point, and to monitor the stress-strain curve. When the curve reaches its peak and then drops, the rock specimen 9 is completely destroyed, and the rock fracture toughness is obtained based on the test data.

[0049] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A rock fracture toughness testing device under true triaxial conditions, characterized in that, The device includes a pressure base, a loading chamber, a cover plate, and a transverse confining pressure loading system. The transverse confining pressure loading system is located inside the loading chamber, which is situated between the pressure base and the cover plate, forming a sealed space. Hydraulic oil is continuously injected into the sealed space to apply longitudinal confining pressure to the rock specimen. An upper loading column is installed at the bottom of the cover plate, which serves as a fracture fulcrum and contacts the rock specimen. A longitudinal load loading plate is provided on the pressure base, and a lower loading column is fixed to the longitudinal load loading plate. The lower loading column is raised under hydraulic drive to apply a fracture load to the rock specimen. The lateral confining pressure loading system is used to apply lateral loads to the rock specimen. It includes a minimum stress loader and a maximum principal stress loader. The minimum stress loader is symmetrically arranged on the left and right sides of the rock specimen and is used to apply the minimum stress of the underground reservoir to the rock specimen. The maximum principal stress loader is symmetrically arranged on the front and rear sides of the rock specimen and is used to apply the maximum stress of the underground reservoir to the rock specimen. The maximum principal stress loader includes a fixed pad, a flexible ultra-thin jack, and a deformation sliding controller. The fixed pad is fixedly connected to the inner wall of the loading chamber. After the flexible ultra-thin jack expands under hydraulic drive, the stress is applied to the deformation sliding controller, thereby transferring it to the rock specimen. The deformation sliding controller has a double-layer foldable structure. The two layers are in contact with each other through ball bearings. The inner layer is connected to the rock specimen and deforms as the rock specimen opens and breaks. The outer layer is connected to the flexible ultra-thin jack and does not deform.

2. The rock fracture toughness testing device under true triaxial conditions according to claim 1, characterized in that, The minimum stress loader includes a fixed pad and a flexible ultra-thin jack. The fixed pad is fixedly connected to the inner wall of the loading chamber, and the flexible ultra-thin jack is installed between the rock specimen and the fixed pad.

3. The rock fracture toughness testing device under true triaxial conditions according to claim 1, characterized in that, The outer layer structure is connected to the top of the inner layer structure by a rotating shaft, and the inner layer structure is fastened to the rock specimen by a fixing strip.

4. The rock fracture toughness testing device under true triaxial conditions according to claim 1, characterized in that, A longitudinal displacement sensor is installed at the bottom of the cover plate, and a strain gauge is installed on the rock specimen. The longitudinal displacement sensor and the strain gauge are connected to an external data acquisition system through a data transmission line to measure the longitudinal and transverse deformation of the rock specimen when it fractures in real time.

5. The rock fracture toughness testing device under true triaxial conditions according to claim 1, characterized in that, The loading chamber is equipped with a pressure valve and a pressure relief valve, both of which are connected to an external hydraulic system.

6. A method for testing the fracture toughness of true triaxial rocks, implemented in the testing apparatus described in claim 2, characterized in that, The steps are as follows: Step 1. Attach strain gauges to the top of the pre-cut groove on the rock specimen, wrap the outside with heat shrink tubing, and use a hot air blower to shrink and tightly wrap the rock specimen; Step 2. Connect the fixing pad to the inner wall of the loading chamber, which is fixed to the pressure base; Step 3. Install the rock specimen into the deformation sliding controller and place it on the lower loading column. Insert the flexible ultra-thin jack between the rock specimen and the corresponding fixing pad. Step 4. Connect the hydraulic lines of the flexible ultra-thin jack and the data transmission lines of the strain gauges to the external data acquisition system, and then install the cover plate onto the loading chamber. Step 5. Start the hydraulic station of the pressurization base to make the upper surface of the rock specimen contact the upper loading column, and then adjust the strain gauge and longitudinal displacement sensor to 0. Step 6. First, start the hydraulic station connected to the flexible ultra-thin jack in the direction of maximum stress to apply a load in the direction of maximum stress to the rock specimen. Then, start the hydraulic station connected to the flexible ultra-thin jack in the direction of minimum stress to apply a load in the direction of minimum stress. Step 7. Start the hydraulic station connected to the loading chamber. After the hydraulic oil is filled into the loading chamber, apply longitudinal confining pressure to the rock specimen. After the longitudinal confining pressure stabilizes, start the test, monitor the stress-strain curve, and obtain the rock fracture toughness based on the test data after the specimen fractures.

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